The present invention relates to a cordless telephone system and, more particularly, to a dual tone multi-frequency (DTMF) signal control apparatus/method for use in a radio fixed unit within the cordless telephone system.
When a radio channel is established between a portable unit and a radio fixed unit connected to an exchange which accommodates push-button (PB) or touch-tone telephones, the audio circuit and line relay of the fixed unit is placed in a connected state, resulting in connection between the exchange and the portable unit. Then, as its bearer dials, the portable unit sends a dial signal to the fixed unit. In response to the dial signal, the fixed unit sends to the exchange a multi-frequency (MF) (usually DTMF) signal corresponding to the dial signal.
The PB receiver of the exchange is so designed as to regard only DTMF signals as valid and to suppress all other signals as noise. Since the audio channel is maintained between the portable unit and the exchange from the time the line relay of the fixed unit becomes connected, not only the DTMF signal but also the dial signal from the portable unit is sent to the exchange. The exchange, judging the dial signal to be noise, actuates its noise suppressor circuit and temporarily disconnects the channel. Because the noise suppressor circuit has a prescribed time constant, the exchange may sometimes fail to detect a DTMF signal sent following the dial signal.
Apart from the above mentioned, there is another problem that, if the bearer of the portable unit dials, and especially if the bearer pushes the re-dial button by mistake in the middle of a conversation, the other party will hear a disagreeable click of the dial signal.